The present invention relates to a fastener system including a threaded fastener having a prevailing torque feature.
Threaded fastener assemblies are used in many applications for aerospace and industrial construction. Such assemblies include threaded bolts or studs and nut members having a matching thread form. In applications subject to vibration or repeated load fluctuations such threaded fastener assemblies have been known to lose their retention torque and to loosen. To inhibit such action, threaded fastener assemblies have been modified to provide a prevailing torque feature to resist loosening and loss of clamp of the workpieces or articles which are joined. Frequently, a prevailing torque feature has been attained by providing one of the mating thread forms with a distorted portion such that, upon engagement between the distorted thread portion and the undistorted thread form, the nut member and the stud or bolt will be additionally held together under a prevailing torque by a thread interference which will resist untorqing and hence resist loosening from vibration.
With standard, non-distorted thread forms, the nut member is free running at low torque values onto the bolt or stud until workpiece engagement and initial clamp or preload on the workpieces. Thereafter the final clamp load attained relative to the magnitude of applied torque is resisted mainly by the inherent friction between the engaged portions of the rotating members under load. This is not the case with threaded fasteners having a distorted, torque prevailing thread form. Here once the distorted thread portion is engaged by the mating threads the nut member is no longer free running and the clamp load of the workpieces being fastened relative to the magnitude of the applied torque is reduced. While the distortion of the thread form is often placed at the outer or trailing end of the nut, this still restricts the free running capability of the nut member onto the stud or bolt and the magnitude of initial preload and final clamp load relative to the applied torque. In addition the mating engagement between the distorted and undistorted threads may be somewhat grip sensitive over the grip range, i.e. with grip range being the variation in total thickness of workpieces, from a maximum grip or total thickness to a minimum grip or total thickness, and to be secured together by a single sized fastener.
The present invention provides a unique nut member which is free running and permits clamping of the workpieces up to a preselected first magnitude of torque. The first magnitude of torque is selected to provide a desired magnitude of initial preload on the workpieces. After this first magnitude of torque is attained in a free running step, a radially compressive force is induced whereby a frictional load is imposed on the threads engaged between the nut member and the bolt or stud. The torque and hence compressive force is increased until a second preselected magnitude of torque is attained at which the installation is completed. The result is a fastened joint with the workpieces held together under a preselected magnitude of final clamp load and with the engaged threads of the nut and bolt or stud of the installed fastener held together under a high compressive load resulting in a high prevailing torque which provides resistance to removal and hence a desired resistance to loosening from vibration or other cyclic loading.
In a preferred form of the present invention the nut member is of a one piece construction with the initial preload being determined by a frangible portion and with the compressive load occurring after the frangible portion has fractured upon attainment of the first magnitude of torque.
The free running capability of the nut member of the present invention is desirable for use in aerospace applications including applications utilizing workpieces made of composite materials.
The unique nut member of the present invention can be utilized with a variety of thread forms including generally standard thread forms available on existing bolts or studs.
The free running capability of the unique nut member of the present invention will be substantially uniformly operative over the grip range of the related fastener.
The free running capability and resultant prevailing torque characteristic of the unique nut member are desirable for many aerospace and non-aerospace applications.
Thus it is an object of the present invention to provide a unique threaded fastener having a prevailing torque characteristic.
It is another object of the present invention to provide a unique threaded fastener including a free running nut adapted to provide a preselected magnitude of initial preload without interference or high frictional loads between the engaging threads and to provide a prevailing torque characteristic after attainment of the initial preload.
In addition it is significant that fasteners used in aerospace applications, with or without a torque prevailing feature, be of a lightweight construction and of minimal size upon final installation.
Thus it is still another object of the present invention to provide a unique threaded fastener including a free running nut which also provides a prevailing torque feature upon installation and which is of a lightweight construction.
Other objects, features, and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, in which: